Perceived loudness is a subjective quantity, and methods for objectively determining perceived loudness have been developed. The hearing threshold of human beings depends on frequency, and furthermore, the frequency dependency of equal loudness contours also varies with the sound pressure level.
Sound level meters can have frequency dependent and level-invariant filters for use in different situations when perceived loudness is to be objectively determined. An example is the widely used A-weighting filter for determining noise level in dB(A). Such filters are relatively simple to construct and give accurate and repeatable measurements. However, the level-dependency of the equal loudness contours is not taken into account, and the use of such fixed filters does therefore not give realistic determinations of perceived loudness.
The international standard ISO 532:1975 Acoustics—Method for calculating loudness level issued by the International Organization for Standardization prescribes measurement of the sound, whose loudness is to be determined, using a single microphone placed at the location of interest. A frequency analysis of the measured sound, preferably a third-octave analysis, forms the basis for determining the loudness using level-dependent equal loudness contours.
ISO 532 only determines loudness of a sound field using a single microphone and assumes that the sound signals at the two ears of a human being are the same. This is true only for a free-field sound with direction of incidence in the median plane, such as frontal incidence, and for diffuse field. It is unclear how loudness should be determined from signals that differ at the ears of a listener.
U.S. Pat. No. 4,631,962 discloses an artificial head measuring system composed of geometric bodies for simulating acoustic properties of a human head. Microphones are disposed in the auditory canals of the artificial head.
U.S. Pat. No. 5,729,612 discloses a method and an apparatus for measuring head-related transfer functions, HRTFs.
The problem to be solved by the invention is therefore to provide a method, which can realistically determine the perceived loudness of any binaural sound signal, i.e. free field, diffuse field, reverberant field or any combination thereof, where the sound can be generated by any combination of point sources and distributed sources with any direction of incidence. The term “binaural” is here used about the situation where a person is subjected to such a sound field and receives corresponding sound signals in his or her left and right ears that are modified by the head-related transfer functions. In general the binaural situation thus also includes e.g. free-field frontal incidence and stimulation using a headphone such as “monotic” stimulation of only one ear and uncorrelated signals in the left and right ears.